Raman spectroscopy study of carbon nanotube peapods excited by near-IR laser under high pressure

A resonant Raman spectroscopy study has been carried out under high pressure, using a diamond anvil cell, on carbon nanotube peapods (${\mathrm{C}}_{60}$@SWNT) synthesized in high yield in our laboratory. The Raman signal was excited by a near IR laser $\left(830\mathrm{nm}\right)$ to avoid photopolymerization of ${\mathrm{C}}_{60}$ and thus obtain the intrinsic vibrational information on the ${\mathrm{C}}_{60}$ molecules in the nanotubes. It is found that the surrounding tubes create an effective pressure on the encapsulated ${\mathrm{C}}_{60}$ due to tube-fullerene interactions, resulting in a shift of the intrinsic ${\mathrm{A}}_{\mathrm{g}}\left(2\right)$ vibrational mode to $1474{\mathrm{cm}}^{-1}$ at ambient pressure. High pressure Raman spectroscopy indicates that ${\left({\mathrm{C}}_{60}\right)}_2$ dimers form near $1\mathrm{GPa}$, and that a further polymerization of ${\mathrm{C}}_{60}$ occurs near $23\mathrm{GPa}$, creating linear chains of covalently linked ${\mathrm{C}}_{60}$ molecules in the tubes.

Figure 1

Typical HRTEM image of a purified bundle of ${\mathrm{C}}_{60}$ peapods, clearly showing ${\mathrm{C}}_{60}$ molecules inside SWNTs. The scale bar is $5\mathrm{nm}$, and the average ${\mathrm{C}}_{60}\text{−}{\mathrm{C}}_{60}$ center-to-center separation is measured to be $0.97\mathrm{nm}$.

Figure 2

Raman spectra for ${\mathrm{C}}_{60}$ peapods using laser excitation at (a) $488\mathrm{nm}$, (b) $830\mathrm{nm}$, and (c) $830\mathrm{nm}$ after photopolymerization by a low intensity $488\mathrm{nm}$ laser, in all cases under ambient conditions. The inset arrow indicates a shoulder peak at $1465{\mathrm{cm}}^{-1}$.

Figure 3

Raman spectra of ${\mathrm{C}}_{60}$ peapods under pressure in the range $0.26–7\mathrm{GPa}$ (a) and the pressure dependence of the ${\mathrm{A}}_{\mathrm{g}}\left(2\right)$ mode frequency (b).

Figure 4

(a) The ambient pressure Raman spectrum for peapod. (b)–(e) Raman spectra for the peapod samples after pressure release from (b) $15.3\mathrm{GPa}$, (c) $17.8\mathrm{GPa}$, (d) $21.5\mathrm{GPa}$, and (e) $23\mathrm{GPa}$, obtained using an excitation wavelength of $830\mathrm{nm}$.